This invention relates in general to the field of telecommunications and fiber optic transmission systems and more particularly to a system and method for differential group delay compensation.
Fiber optics technology and fiber optic transmission systems are revolutionizing telecommunications. The main driving force behind this revolution is the promise of extremely high communications bandwidth. A single beam of modulated laser light can carry vast amounts of information that is equal to literally hundreds of thousands of phone calls or hundreds of video channels. Over the past few years, this technology has advanced at such a pace that the bandwidth capabilities have more than doubled every two years. The bandwidth strides have come about through major milestones, breakthroughs, and improvements in various areas such as fiber optic materials and transmitter devices. As a result, bandwidth capability, which may be expressed in terms of a digital bits per second ("bps") rate, has escalated. In some cases, for example, bandwidth has increased from 500 Mbps to 10 Gbps.
In a fiber optic transmission system, a digital signal is represented by an optical signal by modulating a laser light or rapidly turning a laser light on and off to represent the various "1" and "0" or "on" and "off" values or states of a digital signal. This may be referred to as amplitude modulation. The laser light, or optical signal, is generally emitted from a laser of an optical transmitter. In the frequency domain, this signal includes numerous frequency components spaced very closely about the nominal center frequency of the optical carrier, such as, for example, 193,000 GHz.
An optical signal is transmitted in a fiber optic transmission system using, generally, an optical transmitter, which includes a light source or laser, an optical fiber, an optical amplifier, and an optical receiver. A modulated optical signal arriving at an optical receiver must be of sufficient quality to allow the receiver to clearly distinguish the on-and-off pattern of light pulses sent by the transmitter. Noise, attenuation, and dispersion are a few of the impairments that can distort an optical signal and render the optical signal marginal or unusable at the receiver. The distortion of an optical signal makes it extremely difficult or impossible for an optical receiver to accurately detect or reconstitute the digital signal. This is because distortion nonuniformly broadens, spreads, or widens the various light pulses resulting in such closely spaced pulses or overlapping pulses that the pulses are virtually indistinguishable from one another.
Conventionally, a properly designed optical link or channel can maintain a Bit Error Rate ("BER") of 10.sup.-13 or better. When an optical channel degrades to a BER of 10.sup.-8, a telecommunications system may automatically switch to an alternate optical channel in an attempt to improve the BER. Otherwise, the optical channel must operate at a reduced or lower bandwidth, which harms overall system performance. Dispersion is a major contributor to distortion of an optical signal, which increases the BER of the optical signal or channel. The distortion caused by dispersion generally increases as the bandwidth or data rate increases and as the optical fiber transmission distance increases.
Dispersion has generally been identified as being caused by (1) chromatic dispersion, or (2) Polarization Mode Dispersion ("PMD"). Until relatively recently, chromatic dispersion received the far greater attention because its adverse effects were initially more limiting at the then available bandwidth and data rate that was considered the leading edge in a fiber optic transmission system. Now, it has been recognized that PMD is one of the limiting factors that must be overcome to take telecommunications and fiber optic transmission systems to the next level and to continue with the heretofore rapid increase and expansion of bandwidth and data rates.
Chromatic dispersion occurs when the various frequency components or colors that make up a pulse of laser light travel at different speeds through an optical fiber and arrive at the optical receiver at different times. This occurs because the index of refraction of a material, such as an optical fiber, varies with frequency or wavelength. As a result, the various pulses of light that make up an optical signal are distorted through pulse spreading, making it difficult or impossible to accurately receive and recover the digital data contained in the optical signal.
Some of the major milestones, breakthroughs, and improvements that solved and/or reduced many of the problems caused by chromatic dispersion have included: (1) single-mode propagation, (2) Distributed Feedback ("DFB") laser with narrow output spectra, and (3) the development of low attenuation/modified-dispersion optical fiber. All of these advances have contributed to increased bandwidth by allowing an optical signal to pass through an optical fiber with relatively low or reduced dispersion, and hence, relatively low or reduced optical signal distortion.
Single-mode propagation was achieved through the development of single-mode optical fiber. This optical fiber allows only a single mode of light to propagate through the fiber. The DFB laser provides a light source to use with single-mode optical fibers. The DFB laser produces a light with an extremely narrow distribution of output frequencies and wavelengths. This minimizes the chromatic dispersion problem caused by the fact that different wavelengths travel at slightly different speeds through a fiber. The low attenuation/modified-dispersion optical fiber provides a dispersion-shifted optical fiber that minimizes the speed-vs-wavelength dependency at a specific wavelength, such as 1550 nm.
Unfortunately, no corresponding major milestones, breakthroughs, and improvements have been achieved to solve and/or significantly reduce the substantial problems and limitations caused by PMD. PMD was previously insignificant relative to the other dispersion effects but now is a limiting factor. As a result, PMD now serves as a major limitation to the continued advancement and improvement in bandwidth and data rates for fiber optic transmission systems. PMD causes, among other problems, a first order effect that is referred to as Differential Group Delay ("DGD"). DGD refers to the fact that the two polarization states or modes of an optical signal, which are orthogonal to one another, are delayed relative to each other resulting in a leading polarization signal and a trailing polarization signal. This delay distorts the optical signal and limits the ability of a fiber optic transmission system to operate at a higher bandwidth and data rate.
It is well known that light can be polarized and that, for a given beam of light, this polarization may be expressed in terms of two orthogonal axes that are normal to the axis of propagation. Each of the two principal polarization modes may be expressed as polarization signals. As an optical signal or beam of light propagates through an optical fiber, birefringence causes the two polarization signals to travel or propagate at different speeds. This results in one of the two polarization signals leading the other polarization signal. Thus, there becomes a leading and a trailing polarization signal. As with chromatic dispersion, this speed difference in the two polarization signals causes pulse broadening and restricts the usable bandwidth of each optical carrier.
In many optical fibers, not only is birefringence present, but the birefringence is nonuniform and is randomly varying throughout the optical fiber. The PMD within a given optical fiber changes as a function of time, temperature, and various other factors. This results not only in the two polarization signals traveling at different speeds, but in the continual realigrnment or reorientation of the principal states of polarization. This is because the orientation of the refractive index in the optical fiber randomly or continually changes as the optical signal propagates through the optical fiber. As a result, the light energy present along one such polarization may leak into the other polarization.
This PMD effect can be, for example, on the order of 10-20 picoseconds in a 100 km optical fiber. This is significant when the modulating pulses are 25-100 ps in width, which are common at higher data rates and planned data rates. Because birefringence is randomly varying, a PMD compensator or DGD compensator must monitor and adapt to the changes so as to keep DGD to a minimum.
PMD may be caused by a number of things such as, for example, asymmetrical fiber optic transmission media, mechanical stresses and strains applied to the fiber optic media, and other physical phenomena such as temperature gradients and changes. PMD is not static, but is dynamic and changes over time. For example, an optical splice may change the birefringence of an optical fiber, and hence the PMD and DGD. Similarly, birefringence in an optical fiber varies over time due to various factors such as fiber aging, and due to temperature and pressure changes along the fiber. A fiber installed above ground can exhibit fairly rapid fluctuations in PMD due to temperature and mechanical forces (e.g., wind blowing the fiber). A fiber buried underground can be sensitive to loads such as street traffic or construction work. Also, the fiber may not have a perfect circular cross-section or may have a distorted cross-section because of manufacturing defects or because of mechanical stresses and strains, such as when the fiber is stapled, attached, or fastened during installation. This causes varying delays of the polarization components.
One approach to actively correct for PMD and DGD is the use of mechanical delay lines. Unfortunately, this only allows for continuous delay up to a specified limit allowing for exact DGD compensation. The drawback of this technique is that it requires a mechanical translation device. The speed, cost, and mechanical reliability of these devices are undesirable.
A more common approach uses a fixed delay line by aligning the principal states of polarization to the fast and slow optical axes of polarization maintaining fiber ("PMF"). The drawback of this technique is that it is fixed and can only compensate for exactly one value of DGD when, in fact, we know that birefringence is randomly varying. PMD is statistical in nature so that the amount of induced DGD varies with time. If the value of DGD differs from the fixed delay in the PMF, the PMDC will only partially compensate for the adverse effects of DGD.
The effect of PMD is one of the dominant limitations to overcome in deploying OC-192 transport systems in fiber optic transmission systems. Furthermore, the deployment of OC-768 transport systems will not be possible without active PMD Compensation ("PMDC") devices. Fixed delay DGD compensation techniques and mechanically implemented techniques are limited and will not be sufficient for higher bandwidth systems.